This application claims priority from Japanese Patent Application No. 016518/2000, filed Jan. 26, 2000, the entire disclose of which is incorporated herein by reference.
1. Technical Field of the Invention
The present invention relates to an apparatus and method for cutting ingots such as single crystal ingots of SiC etc., used in hard electronics.
2. Prior Art
Hard electronics generally means solid state electronics based on wide-gap semiconductors with physical properties better than those of silicon, such as SiC and diamond, which have harder specifications than those of silicon. The band gaps of SiC and diamond used in hard electronics are in the range of 2.5 to 6 eV compared to the 1.1 eV of silicon.
The history of semiconductors began with germanium which was succeeded by silicon with a greater band gap. A large band gap brings with it a greater chemical bonding force between the atoms that compose a substance. Therefore, physical properties required for hard electronics, such as material hardness, insulation breakdown voltages, carrier saturation drift velocities and thermal conductivities are much better than those of silicon. For example, the Johnson index for a high-speed, large-output device is one of the performance indexes used in hard electronics. As shown in FIG. 1, if the index is assumed to be 1 for silicon, those of the semiconductors used in hard electronics are a hundred to a thousand times greater.
Therefore, semiconductors based on hard electronics are considered to be very hopeful as replacements for conventional silicon semiconductors in various fields such as high energy electronics typically used for power devices, electronics for information technologies based mainly on millimeter waves and microwave telecommunications and electronics for extreme environments including nuclear power, geothermal heat and space technologies.
Of the various hard electronics materials, power devices using SiC have reached the most advanced stage of research. However, even though SiC devices are at the leading edge of research and development, because this material has a strong chemical bonding force and is very hard, there are problems in the manufacture of devices made of SiC material, and conventional technologies used for processing silicon cannot be directly applied.
That is, to manufacture a device from an ingot of single-crystal SiC, the ingot must be cut into flat wafers in the same way as is done conventionally. According to the conventional technology for processing silicon, the ingot is cut using either (1) an outer edge cutter, (2) an inner edge cutter or (3) a wire saw.
The outer edge cutter is shown typically in FIG. 2. A thin disk-shaped cutter with a cutting edge 2 is rotated at a high speed about its center shaft 2a, and its outer edge cuts the ingot 1. This type of cutter has been used conventionally to cut single crystals of SiC. However, with this type of cutting means, if the diameter of the ingot is 3 inches (about 75 mm), the thickness of the cutting edge is about 0.8 mm and the diameter of the disk is about 8 inches (about 200 mm). Therefore the thickness of the material lost in cutting (corresponding to the edge thickness+runout) is larger than the thickness of the product (about 0.3 mm). That is, the problem concerns the loss of a large amount of expensive single crystal SiC. In addition, the diameter of a single crystal SiC ingot has been increased to 4 inches or more (about 100 mm or more) as there is a demand for large devices and the manufacturing technology has advanced. In this case, the diameter of the cutting disk is about 10 inches (about 250 mm) and the size of the cut is about 1.0 mm, so the losses become much greater.
In addition, as the diameter of the cutting disk is large, another problem is that saw marks are produced on the cut surface.
The inner edge cutter is shown schematically in FIG. 3. A thin cutting disk 3 with a hole 3a at the center is rotated at a high speed, and the ingot 1 is cut by grinding material electrolytically deposited on the inner periphery. The cutting disk 3 is a metal plate with a thickness as small as 0.2 to 0.3 mm, and the outer periphery is supported by another ring member (not illustrated) in order to keep the plate flat.
With this type of cutting means, the cutting losses can be reduced in the case of an easily cut silicon ingot, because the cutting edge is thinner than the cutting edge 2 in FIG. 2. However, when a hard crystal of SiC is cut, the life of the cutting edge is short because there is only one layer of electrolytically deposited grinding particles. So there is a problem of short replacement intervals. Also, the mounting structure of the cutting disk 3 is complicated, and the installation needs skillful personnel, so that the replacement work is time-consuming. In addition, there is another problem because the operating efficiency of the cutting device is low.
With the wire saw, as illustrated in FIG. 4, a fine wire 4, 0.2 to 0.3 mm in diameter, is stretched between the guide pulleys 4a and pulled across in an endless-manner. The ingot is cut by slurry containing grinding grains supplied between the ingot 1 and the wire 4. Because this type of cutting method cuts slowly with the help of a slurry, normally a number of wafers (4 to 8 wafers) are cut simultaneously by one length of wire 4 as shown in FIG. 4.
Although this cutting means causes only a small amount of cutting losses, when a hard single crystal of SiC is cut, the wire is rapidly consumed and breaks frequently. In particular, the wire is often cut at the outer periphery of the ingot 1 because of considerable vibrations. Once the wire breaks, the single crystal of SiC being cut is totally lost, so the large loss of an ingot is the problem. Also, a single crystal sic ingot is hard and difficult to cut, so that a large amount of slurry is required, resulting in a high cost.
As described above, when a single crystal of SiC is cut, the following requirements must be satisfied.
(1) The hard, refractory single crystal of SiC must be cut efficiently.
(2) Cutting means must be applicable to a crystal with a diameter as large as 4 inches.
(3) The width of the cut should be small so that only a small amount of expensive single crystal SiC is lost during cutting.
(4) The warping of the cutting plane (that is, of the entire wafer) must be small. This warping requirement is particularly important because warping cannot be corrected during subsequent lapping etc., and the maximum amount of warping should be 30 xcexcm or less.
(5) No saw marks.
(6) Processing damage to the crystal should be minimal.
(7) The running costs must be low.
(8) The manpower required should be low.
The present invention aims at solving the various problems and satisfying demands. In other words, an object of the present invention is to provide an apparatus and method for cutting ingots such that a large, hard and refractory ingot can be cut efficiently with a small amount of cutting losses, a small degree of warping and thickness irregularity on the finished surface, small roughness of the cut surface, minimal damage to the crystal during processing, low operating costs, and small manpower requirements.
The ingot cutting apparatus offered by the present invention is provided with a thin strip-shaped grindstone (12), a tensioning mechanism (14) that applies a tension to the above-mentioned strip-shaped grindstone to keep the grindstone flat, a reciprocating device (16) to move the strip-shaped grindstone backwards and forwards in the longitudinal direction, and a cutting device (18) that moves the strip-shaped grindstone in the direction of the diameter of the cylindrical ingot (1).
In addition, according to the present invention, a method of cutting ingots is provided. In the method, a tension is applied to thin strip-shaped grindstone (12) to maintain the grindstone flat, the strip shaped grindstone is moved backwards and forwards in the longitudinal direction, the strip-shaped grindstone is moved in the radial direction of the cylindrical ingot (1) and the ingot is cut.
According to the above-mentioned apparatus and method of the present invention, because a strip-shaped grindstone (12) is moved backwards and forwards longitudinally while cutting a cylindrical ingot (1), the ingot can be cut efficiently even if it is large in diameter and hard to cut. Compared to conventional means that use an outer or inner cutting edge disk cutter, the cutting tool (strip-shaped grindstone) is smaller and cheaper, so the running cost can be reduced. In addition, as the strip-shaped grindstone is tensioned and maintained flat, a thin strip-shaped grindstone with a thickness for example, of 0.2 to 0.3 mm can be used, so that the runout of the grindstone can be reduced. Therefore, the cutting losses can be decreased, and the warping or uneven thickness of the finished surface can also be decreased. Furthermore, because the strip shaped grindstone is more resistant to breakage than a wire, the loss of an expensive ingot (for instance, of a single crystal of SiC) can be greatly reduced.
According to a preferred embodiment of the present invention, the tensioning mechanism (14) is composed of a pair of fixing components (14a) that are attached to both ends of the strip-shaped grindstone (12), and a tensioning component (14b) that pulls the above-mentioned fixing components in the longitudinal direction of the strip-shaped grindstone. The reciprocating device (16) is comprised of a double-action bed that drives the above-mentioned tensioning mechanism (14) backwards and forwards in the horizontal or vertical direction. The cutting device (18) is composed of a moving device that holds the ingot (1) and drives it in a direction parallel to the plane of the strip-shaped grindstone.
This configuration simplifies the structure of the apparatus, reduces machine failures, increases the operating efficiency, reduces running costs, can be easily automated, and saves manpower.
Moreover, the above-mentioned tensioning mechanism (14) should preferably support a number of strip-shaped grindstones (12) mounted parallel to each other. Such a configuration as described above provides for multiple cutting (the ingot is cut at a number of locations simultaneously) using a plurality of strip-shaped grindstones, so the configuration can also increase the rate of cutting.
Also, the strip-shaped grindstone (12) is a metal-bonded grindstone, and is provided with at least one pair of electrodes (23) arranged on both sides of the ingot in the radial direction, separated from both surfaces of the metal-bonded grindstone, a means (22) for applying a voltage to supply DC voltage pulses to the above-mentioned electrodes with the metal-bonded grindstone as the positive electrode, and a means (24) of feeding processing fluid to supply a conducting processing fluid (25) between the metal-bonded grindstone and the above-mentioned electrodes. A minimum of one pair of electrodes (23) are arranged adjacent to both surfaces of the metal-bonded grindstone on both sides of the ingot in the radial direction. DC voltage pulses are applied to the electrodes with the metal-bonded grindstone as the positive electrode, and at the same time, conducting processing fluid (25) is supplied between the metal-bonded grindstone and the electrodes, the cylindrical ingot is cut by the metal-bonded grindstone, and simultaneously, both surfaces of the grindstone are dressed electrolytically on both sides thereof.
Using the apparatus and methods, so-called electrolytic in-process dressing grinding (ELID grinding) can be carried out, wherein an ingot is cut while both surfaces of the metal-bonded grindstone are electrolytically dressed. As a result of the electrolytic dressing, the grinding grains are sharpened, so that even a hard single crystal SiC ingot can be cut efficiently. In addition, since the surface of metal-bonded grindstone can be sharpened with a high degree of accuracy by the above-mentioned electrolytic dressing, microscopic grinding grains can be used and the cut surface can be finished to give an excellent flat surface with a near-mirror surface finish. Furthermore, the amount of subsequent processing (polishing) can be greatly reduced, and also processing damage to the crystal can be minimized.
The above-mentioned strip-shaped grindstone (12) is composed of a strip of metal (13) and a metal-bonded grindstone (12a) formed on the edge thereof by electric casting. With this configuration, a metal-bonded grindstone that can withstand the tension needed to keep the grindstone flat can be easily manufactured.
Other objects and advantages of the present invention will be revealed in the following description referring to the attached drawings.